This invention relates to electric switches suitable for use in underground power distribution systems and, more particularly, to gas-insulated switches utilizing electronegative gas.
Submersible power switches for use in underground distribution systems are known. These switches are used to provide means for both interrupting and completing underground power distribution circuits. Prior art underground distribution switches generally take two forms. The first of these two forms is a vacuum interrupter switch wherein the distribution switch contact or contacts engage and disengage one another in an atmospherically evacuated or vacuum environment. The vacuum environment is relied upon to provide insulation between the electrodes and to provide rapid extinction of the arc drawn between the electrodes during a switch-opening operation. However, it is known that a vacuum type interrupter switch presents substantial problems such as: (i) maintaining the vacuum integrity; (ii) providing sufficient mechanical clearance between the contact elements of the electrodes in the restricted vacuum envelope so as to permit adequate electrode contact in the closed-circuit position while assuring BIL (Basic Impulse Level) integrity in the open-circuit position; and (iii) premature circuit interruption or current-chopping which generates over voltages dangerous to the insulation integrity of the vacuum switch and the associated external electrical devices.
The second form of prior art distribution switches is the oil-filled interrupter switch wherein a volume of oil surrounds the switching electrodes within a housing. However, the attendant arcing in these oil-filled switches not only generates gas pressures within the housing which can rupture the housing but it also produces explosive gases which when combined with the combustible oil may result in a fire hazard. Additionally, these prior art oil-filled switches require periodic maintenance such as testing or changing of the oil medium.
The circuit-interrupting properties of a gas-filled circuit interrupter utilizing an electronegative gas have been known to the art as exemplified in the patent to Lingal, U.S. Pat. No. 2,757,261. An electronegative gas, such as sulphur hexafluoride (SF.sub.6) poses several properties which make it eminently suitable for use in a circuit interrupter. For example, SF.sub.6 is chemically and physiologically inert and non-flammable. Further, at atmospheric pressure, the dielectric strength of SF.sub.6 is approximately 2.5 times that of air. Perhaps the most valuable intrinsic property of SF.sub.6 is its arc-quenching capability. In a simple break interrupter, i.e., one wherein the switching is accomplished in still air, approximately 100 times as much current can be extinguished using SF.sub.6 as compared to air, and when this electronegative gas is blown through the arc, even at low velocities, the effectiveness compared to a simple break, air interrupter is further multiplied to hundreds of times. The Lingal patent teaches the art that the current or voltage interrupting ability of the electronegative gas is considerably improved by simply increasing the pressure of the gas in the switch chamber; and that the interrupting ability increases almost directly with the absolute pressure.
Circuit breakers utilizing the optimum arc-extinguishing ability of an electronegative gas, such as SF.sub.6, as taught by the Lingal patent, are known to the art as exemplified in U.S. Pat. No. 3,749,869. These circuit breakers have increasingly been used at high gas densities and high gas pressures in order to optimize the desirable effects as taught in the Lingal patent. As well known by those skilled in the art, however, when an electronegative gas such as SF.sub.6 is utilized at high pressure, there results the hazard that at low ambient temperatures, the gas may become liquefied, and this necessarily will cause a drop in its operating pressure. The drop of operating pressure is, of course, undesired, as the possibility exists that the circuit breaker will be incapable of interrupting its current and voltage ratings.
As illustrated in the U.S. Pat. No. 3,749,869, the prior art attacks the above-described liquefication problem by resorting to means for adding external heat to the gas as by including a heater element with the structure of the circuit breaker. In this manner, workers in the art have utilized what heretofore has been regarded as the optimum benefits of an electronegative gas while avoiding the highly undesired effects of liquefication. However, the added heating means also adds a continuing maintenance and reliability problem.
Another problem associated with prior art high-pressure gas switches is the difficulty in maintaining the relatively high pressure levels within the enclosed chamber. In order to maintain the high-pressure levels, particularly for underground or unattended installations, it has been necessary to provide either elaborate sealing means as part of the enclosed chamber, or gas-filled, pressurized containers external to the enclosed chamber and in communication therewith.
The gas-insulated switch, in accordance with the present invention, overcomes the problems associated with prior art switch gear by combining the concept of electronegative gas switching in a unique distribution switch configuration, thereby to provide a compact and relatively maintenance-free distribution switch which is particularly suited to installation below ground and at ground level.